Process and apparatus for the low-temperature fractionation of air

Abstract

The process and the apparatus serve for the low-temperature fractionation of air. Feed air ( 1 ) is introduced into a first rectification column ( 3 ). A transfer fraction ( 6, 7 ) of density ρ is withdrawn in the liquid state from a reservoir ( 24, 16 ) within the first rectification column ( 3 ), expanded ( 14, 14 a , 18 ) and fed to a further process step ( 5, 23 ). The liquid level in the reservoir ( 24, 16 ) is in this case at a first level h1 and is at a first pressure p1. The expanded transfer fraction is fed to the further process step ( 5, 23 ) at a second, higher level h2 (h2 >h1) and at a second, lower pressure (p2 <p1). The difference between the two pressures Δp=p1−p2 is less than the hydrostatic pressure (Phydr=ρ·g·[h2−h1]) caused by a liquid column of the transfer fraction between the first level and the second level: Δp=p1−p2<ρ·g·[h2−h1] (g: acceleration due to gravity). The expansion ( 14, 14 a, 18 ) is carried out in such a manner that the gas bubbles produced on expansion decrease the density of the transfer fraction to the extent that the pressure difference Δp is sufficient for feeding the transfer fraction to the further process step ( 5, 23 ).

What is claimed is:  
     
       1. A process for the low-temperature fractionation of air, comprising: 
       introduced feed air into a first rectification column;  
       withdrawing a transfer fraction of density ρ in the liquid state from a reservoir within the first rectification column, wherein a liquid level in the reservoir is at a first level h1 and is at a first pressure p1;  
       expanding transfer fraction such that gas bubbles produced on expansion decrease the density of the transfer fraction to the extent that the pressure difference Δp is sufficient for feeding the transfer fraction to the further process step;  
       feeding the expanded transfer fraction to the further process step at a second, higher level h2 (h2 >h1) and at a second, lower pressure (p2<p1), wherein the difference between the two pressures Δp=p1−p2 is less than the hydrostatic pressure (Phydr=ρ·g·(h2−h1) caused by a liquid column of the transfer fraction between the first level and the second level:  
       
         
           Δp=p1−p2<ρ·g·[h2−h1] 
         
       
        (g: acceleration due to gravity).  
     
     
       2. A process according to claim  1 , further comprising subcooling the transfer fraction by indirect heat exchange upstream of the expansion. 
     
     
       3. A process according to claim  2 , wherein the subcooling is carried out such that the transfer fraction, immediately upstream of the expansion, comprises a liquid. 
     
     
       4. An apparatus for the low-temperature fractionation of air, comprising: 
       a first rectification column having a reservoir for a liquid transfer fraction;  
       a liquid line connected to the reservoir in the first rectification column and to a further vessel and comprising an expansion valve;  
       where:  
       a liquid level in the reservoir is at a first level h1 and is at a first pressure p1,  
       in the further vessel at the connection point between liquid line and further vessel a second pressure (p2) prevails in the operating case,  
       the connection point between liquid line and further vessel is disposed at a second, higher level h2 (h2>h1),  
       the difference between the two pressures Δp=p1−p2 in the operating case is less than the hydrostatic pressure (Phydr=ρ·g·(h2−h1) caused by a liquid column of the transfer fraction between the first level and the second level:  
       
         
           Δp=p1−p2<ρ·g·(h2−h1)  
         
       
        (g: acceleration due to gravity  
       wherein the expansion valve is disposed such that gas bubbles produced on expansion decrease the density of the transfer fraction during the expansion to the extent that the pressure difference Δp is sufficient for feeding the transfer fraction to the further vessel.  
     
     
       5. An apparatus according to claim  4 , further comprising a heat exchanger for cooling the transfer fraction by indirect heat exchange disposed in the liquid line upstream of the expansion valve. 
     
     
       6. An apparatus according to claim  5 , wherein the heat exchanger is disposed so that the transfer fraction, immediately upstream of the expansion valve, comprises a liquid.